1. Field of the Invention
The present invention relates to a technique for relative positioning between a mask pattern and a photosensitive substrate, which technique is applied to an exposure apparatus used in a photolithography process for exposing the mask pattern onto the photosensitive substrate in manufacturing, e.g., semiconductor elements and, more particularly, to a technique for detecting the mark pattern on the photosensitive substrate.
2. Related Background Art
In the photolithography process for manufacturing, e.g., semiconductor elements, liquid crystal display elements, or thin-film magnetic heads, an exposure apparatus is used to transfer an image of a photomask or reticle (to be simply referred to as a reticle hereinafter) having a transfer pattern onto a photoresist-coated wafer (or a photosensitive substrate such as a glass plate) in accordance with a projection exposure method through a projection optical system or proximity exposure method.
In this exposure apparatus, positioning (alignment) between a reticle and a wafer must be performed with high precision prior to exposure. A position detection mark (alignment mark) obtained by exposure, transfer, and etching in the previous process is formed on the wafer. The accurate position of the wafer (i.e., a circuit pattern on the wafer) can be detected by detecting the position of this alignment mark.
In recent years, there has been proposed a method of forming a one-dimensional or two-dimensional grating mark on a wafer (or a reticle), projecting two coherent beams symmetrically inclined in the pitch direction on the grating mark, and causing two diffracted light components generated from the grating mark in the same direction to interfere with each other to detect the position and offset of the grating mark in the pitch direction, as disclosed in Japanese Patent Application Laid-open Nos. 61-208220 (corresponding U.S. Pat. No. 4,828,392; to be referred to as reference (A) hereinafter), 61-215905 (corresponding U.S. Pat. No. 4,710,026; to be referred to as reference (B) hereinafter), and the like. Reference (A) discloses a homodyne scheme in which two symmetrical coherent beams have the same frequency, while reference (B) discloses a heterodyne scheme in which a predetermined frequency difference is present between two symmetrical coherent beams.
Heterodyne schemes in which position detection devices of these schemes are applied to a TTR (Through-The-Reticle) alignment system and a TTL (Through-The-Lens) alignment scheme are proposed in Japanese Patent Application Laid-open Nos. 2-227602 (corresponding U.S. Ser. No. 483,820 filed on Feb. 23, 1990, now U.S. Pat. No. 5,489,968 issued Feb. 6, 1996; to be referred to as reference (C) hereinafter), 3-2504 (corresponding U.S. Pat. No. 5,118,953; to be referred to as reference (D) hereinafter), and the like. In the heterodyne schemes disclosed in these references (C) and (D), an He--Ne laser beam is simultaneously incident on two acousto-optic modulators (AOMs), and the AOMs are driven by high-frequency drive signals (one drive signal: 80 MHz; the other drive signal: 79.975 MHz) having a frequency difference of, e.g., about 25 kHz, thereby imparting the frequency difference of 25 kHz to diffracted beams emerging from these AOMs. These two diffracted beams serve as a pair of incident beams for irradiating a grating mark on a wafer or reticle at a predetermined crossing angle.
In the heterodyne scheme, the frequency difference (25 kHz) between the two incident beams is given as a reference AC signal, a phase difference between the reference AC signal and a signal obtained by photoelectrically detecting an interference light beam (beat light beam) of two diffracted light components generated from the grating mark is measured, and this phase difference is detected as a position offset (positional deviation) amount from the reference point in the pitch direction of the grating mark.
According to the heterodyne scheme described above, when the two incident beams for illuminating the grating mark have better monochromaticity, the detection precision in position offset, i.e., a resolution can increase. Position detection and positioning on the nanometer order can be performed. Excellent monochromaticity in the two incident beams indicates that the phase on the wavelength order between various diffracted light components generated by the grating mark tends to be sensitively changed by asymmetry of the grating marks, a resist layer, and the like.
An influence of the resist layer cannot be inevitably avoided in wafer alignment in an exposure apparatus. Unless a special technique for locally removing a resist from a mark portion is used, or unless an optical mark detection technique is given up, this problem is left unsolved.
A heterodyne scheme capable of reducing the influence of the resist layer and an influence of asymmetry in the sectional shape of the marks and allowing more accurate position detection is proposed in Japanese Patent Application Laid-open No. 6-82215 (corresponding U.S. Ser. No. 091,501 filed on Jul. 14, 1993; to be referred to as reference (E) hereafter). According to a technique disclosed in this reference (E), a plurality of beams having different wavelengths, or a white beam is used, and two diffracted beams obtained upon irradiation of the plurality of beams or the white beam on a stationary diffraction grating are incident on the first AOM. 0th-, +1st- and -1st-order beams diffracted by the first AOM are relayed to cross each other in the second AOM, thereby obtaining a pair of incident beams having the first wavelength and a pair of incident beams having the second wavelength. These two pairs of incident beams are simultaneously projected onto the grating mark on the wafer.
In this case, an interference beat light beam generated by the grating mark and photoelectrically converted include the first wavelength component and the second wavelength component. These components are photoelectrically detected in the form of a sum as a light amount on the light-receiving surface of a photoelectric element. For this reason, the mutual phase differences of the interference beat light beams of the respective wavelength components caused by the influence of the thin-film interference of the resist layers and the influence of asymmetry in the sectional shape of the marks can be averaged in terms of light intensity. Therefore, more accurate position detection can be performed.
Regardless of the homodyne and heterodyne schemes, a light source suitable for obtaining a multi-wavelength illumination light beam is generally selected from light sources having a high coherency and a sufficiently large light intensity, such as a gas laser light source or a semiconductor laser light source. For this reason, wavelengths selected for a multi-wavelength light beam in the conventional arrangement are determined such that the oscillation center frequency is shifted by an appropriate amount, e.g., 20 nm to 40 nm from practical light sources (e.g., laser light sources having excellent records of performance).
The surfaces of the grating marks on the wafer are coated with a resist layer having an almost predetermined thickness (e.g., about 0.5 .mu.m to 1.5 .mu.m). This thickness varies on a given central value. In addition, the thickness of the resist layer varies depending on the positions on the wafer. The sectional shape (small step differences of the grating grooves) of the grating marks slightly changes depending on the positions on the wafer in accordance with a wafer process. The total amplitude reflectance of the portion including the grating marks and the resist layer on their surfaces often greatly varies with respect to an illumination light beam which is converted as a multi-wavelength illumination light beam with a specific wavelength component.
Wavelength selection for forming a multi-wavelength illumination light beam cannot always be optimized so as to achieve high-precision position detection for a grating mark whose amplitude reflectance can greatly vary.
Assume that an illumination light beam used for position detection is formed into a beam having a plurality of wavelengths or a beam having a predetermined wavelength bandwidth, and that interference light beams including a plurality wavelength components generated by the grating mark are simultaneously received by a single photoelectric element. In this case, if the illumination light beam includes a high-intensity wavelength component, the interference light beam from the grating mark is increased at this wavelength component, thus often posing a problem to obtain an averaging effect. In addition, even if the intensities of the wavelength components in the illumination light beam are identical to each other, large differences may occur for the wavelength components of the interference light beam from the grating mark depending on the surface state (e.g., the irregularity in thickness of the resist and the degree of asymmetry of the grating marks) on the photosensitive substrate such as a wafer.
Even if an interference light beam having a plurality of wavelength components generated by the grating mark is received by a single photoelectric element, high precision of position detection cannot always be obtained depending on the surface state of the substrate.
A heterodyne system in which the influence of a resist layer and the influence by the deformation of the shape (such as the asymmetry of the cross-sectional shape) of a grating mark are reduced to thereby make more accurate position detection possible is also proposed by U.S. Pat. No. 5,160,849 (H).
In this publication (H), there is disclosed a technique of finding a first position offset amount of the grating mark measured on the basis of the photoelectric signal of the interference beat light of .+-.1st-order diffracted lights created perpendicularly from a grating beam and a second position offset amount of the grating mark measured on the basis of 0-order diffracted light and .+-.2nd-order diffracted lights created from the grating beam, and selecting one of the two positional deviation amounts.
Also in a position detecting system of the homodyne type, as disclosed in Japanese Patent Application Laid-open No. 61-208220 (I), there is proposed an attempt to know the asymmetry of a diffraction grating by comparing the magnitudes of the light intensities of a plurality of high-order diffracted lights created from a diffraction grating on a substrate.
However, in the system of the publication (I) wherein as in the prior art, a variation in the distribution by the each of the intensities of high-order diffracted lights created from a grating mark by the application of an illuminating beam of a single wavelength is presumed, a number of data bases for making various conditions such as the optical non-uniformity of a resist layer covering the grating mark and the asymmetry of the cross-sectional structure of the grating mark and the variation in the distribution of the intensities of the high-order diffracted lights correspond to each other become necessary.
Also, in the system as disclosed in the publication (H) wherein one of the first position offset amount measured on the basis of the coherent light of 1st-order diffracted components from the grating mark and the position offset amount measured on the basis of the coherent lights of 0-order and 2nd-order diffracted components, there has been the problem that the deterioration of the position offset detection accuracy by the optical non-uniformity of the resist layer and the influence of the asymmetry of the cross-sectional structure of the grating mark cannot be synthetically suppressed (the improvement in the accuracy becomes limited).
Also, in connection with the technique of the publication (H), it would occur to mind to average the measured first position offset amount and second position offset amount with weight given thereto in conformity with the intensities (the degrees of modulation) of the respective interference lights, but it may sometimes be impossible to attain an improvement in accuracy simply by the averaging having added thereto the simple weight conforming to the degrees of modulation of the interference lights from the grating mark on a wafer. This is considered to be caused by the fact that the amplitude reflectance of the surface portion of the wafer including the grating mark for the illuminating beam changes variously for each wafer and at each location (shot area) on the wafer.